Because you don't know what is going to be done with it, quite simply. We live in an age where digital manipulation is not only (relatively) easy and cheap, but getting to the point where it's difficult to tell that anything has been done at all. Now, I'm not saying that the government is going to digitally manipulate phone calls made by you and pictures of you to frame you for some crime (although they could), but digital information isn't exactly the most secure.
Would you allow the police to place CCTV's all through-out your house and neighbourhood, workplace, grocery stores, gyms, etc. just so that they could make sure you weren't being robbed, or assaulted? Now what if they said the reason was to make sure you weren't going to rob or assault someone else, or to better protect a potential victim if you were to ever do that. I highly doubt many people would agree to this, honest or not.
As far as the last sentence goes, it's my belief that the government has far overstepped its bounds in regards to internet surveillance and information security. Going back to my last example, I'd be pretty livid if I found out that the government had installed secret cameras and recorders all through-out my life and were saving all data "in case it's needed in the future." In my opinion, it's the same thing, and I can see a lot more going wrong with it than the good it can provide society. They didn't need to monitor everything about everyone all the time before, they don't need to do it now.